Plant viruses and other virus-like particles are utilized in expanded roles as multi-facet nanosized building blocks for directing cell growth and differentiation. Plant viruses are isolated in high purity with batch to batch consistencies in time-honored fashion at low costs. The surface properties of the virus are adjusted through chemical or genetic modifications to incorporate new biologically relevant functional groups. Furthermore, the symmetrical arrangement of the viral proteins make the viral particles attractive scaffolds for displaying identical copies of the functional groups for applications in stem cell cultures, electronics, catalysis, drug/gene delivery, imaging, and immunotherapy.
An argument for using viruses as a biomaterial lies on the premise that structurally ordered functional groups recruit different cellular responses compared to unordered ligands. For example, influenza virus attaches to erythrocytes through multiple binding between hemagglutinin and sialic acid, and some animal viruses display integrin binding sites in a pentameric motif to promote cell internalization. The adhesion force associated with the clusters of integrin binding motifs can be 7-fold stronger over non-clustered ligand-receptor interactions. In cell signaling, the integrin receptors, which are targets for the RGD peptide, form dynamic clusters which are crucial in cell adhesion, motility, as well as echano-transduction, which can all invariably affect stem cell differentiation.
TMV is one of the simplest viruses known. Each viral particle consists of 2130 identical protein subunits arranged in a helical motif around a single stand of RNA to produce a hollow protein tube. The internal and external surfaces of the protein consist of repeated patterns of charged amino acid residues, such as glutamate, aspartate, arginine, and lysine. The rod like TMV is 300 nm in length and 18 nm in diameter. The chemistry of TMV has been studied extensively, and it has been previously demonstrated that coating surfaces with TMV and another plant virus, Turnip yellow mosaic virus (TYMV), enhanced mesenchymal stem cell differentiation towards bone-like phenotype.
Studies have indicated that the coat protein of Tobacco mosaic virus (TMV) can tolerate up to 25 amino acid insertions near its carboxy terminus. By inserting cell-binding sequences to the virus coat protein, specific bio-functionalities can be engineered for use in tissue engineering. The advantages of using TMV or other plant virus particles are multi-fold. First, the structural features and the size range of the virus could be envisioned as highly stabilized, macromolecular extracellular matrix mimetic. The natural extracellular matrix (ECM) proteins play important roles in guiding cell adhesion, migration, proliferation and stem cell differentiation. However these ECM proteins difficult to obtain high yields in cost-effective manner, but the virus-based ECM mimics are routinely purified in high purity (>99%) in time-honored manner. Second, previous studies with plant virus coated substrates demonstrated that the coating of surfaces with native and phosphate-modified virus particles accelerated stem cell differentiation towards bone-like tissues from 21 days of culture to 14 days. The accelerated differentiation process was marked by increased expression levels of key osteogenic markers and an important growth factor known as bone morphogenetic protein-2 (BMP-2). Although the role of TMV coated substrates and BMP-2 has not been fully elucidated, these early studies indicate that the virus-mediated differentiation is not simply based on virus-cell interactions but multiple interactions between the osteoinduction factors (dexamethasone, beta-glycerophosphate and ascorbic acid), serum/cytokine soluble factors, and surface topography.